Transparent resin films are used in a variety of optical applications. For example, a number of different optical elements in Liquid Crystal Displays (“LCDs”) may be formed from resin films. The structure of LCDs may include a liquid crystal cell, one or more polarizer plates, and one or more light management films. Liquid crystal cells are formed by confining liquid crystals such as vertically-aligned (VA), in-plane switching (IPS), twisted nematic (TN) or super twisted nematic (STN) materials between two electrode substrates. Polarizer plates are typically a multi-layer element comprising resin films. In particular, a polarizer plate can comprise a polarizing film sandwiched between two protective cover sheets that comprise a low birefringence protective polymer film.
Polarizing films are normally prepared from a transparent and highly uniform, amorphous resin film that is subsequently stretched to orient the polymer molecules and then stained with a dye to produce dichroic film. An example of a suitable resin for the formation of polarizer films is fully hydrolyzed poly(vinyl alcohol) (PVA). Because the stretched PVA films used to form polarizers are very fragile and dimensionally unstable, protective cover sheets are normally laminated to both sides of the PVA film to offer both support and abrasion resistance.
Protective cover sheets used in polarizer plates are required to have high uniformity, good dimensional and chemical stability, and high transparency. Originally, protective coversheets were formed from glass, but a number of resin films are now used to produce lightweight and flexible polarizers. Many resins have been suggested for use in protective cover sheets including cellulosics such as cellulose esters, acrylics such as poly(methyl methacrylate), cyclic polyolefin, polycarbonates, and sulfones. However, acetyl cellulose polymers are most commonly used in protective cover sheets for polarizer plates. Polymers of the acetyl cellulose type are commercially available in a variety of molecular weights as well as the degree of acyl substitution of the hydroxyl groups on the cellulose backbone. Of these, the fully substituted polymer, triacetyl cellulose (TAC) is commonly used to manufacture resin films for use in protective cover sheets for polarizer plates.
The cover sheet normally requires a surface treatment to insure good adhesion to the PVA dichroic film. When TAC is used as the protective cover film of a polarizer plate, the TAC film is subjected to treatment in an alkali bath to saponify the TAC surface to provide suitable adhesion to the PVA dichroic film. The alkali treatment uses an aqueous solution containing a hydroxide of an alkali metal, such as sodium hydroxide or potassium hydroxide. After alkali treatment, the cellulose acetate film is typically washed with weak acid solution followed by rinsing with water and drying. This saponification process is both messy and time consuming.
U.S. Pat. No. 2,362,580 describes a laminar structure wherein two cellulose ester films each having a surface layer containing cellulose nitrate and a modified PVA is adhered to both sides of a PVA film. JP 06094915A discloses a protective film for polarizer plates wherein the protective film has a hydrophilic layer which provides adhesion to PVA film. Commonly-assigned, copending U.S. patent application Ser. No. 10/838,841, filed May 4, 2004 describes a guarded protective cover sheet having a removable, carrier substrate and a cover sheet comprising a low birefringence protective polymer film and a layer promoting adhesion to poly(vinyl alcohol) on the same side of the carrier substrate as the low birefringence protective polymer film which eliminates the need for the saponification process.
Protective cover sheets may be a composite or multilayer film including other functional layers (herein also referred to as auxiliary layers) such as an antiglare layer, antireflection layer, anti-smudge layer, compensation layer, or antistatic layer. Generally, these functional layers are applied in a process step that is separate from the manufacture of the low-birefringence protective polymer film, but may be later applied to form a composite film. A functional or auxiliary film may combine functions of more than one functional layer, or a protective polymer film may also serve the function of a functional layer.
For example, some LCD devices may contain a low birefringence protective polymer film that also serves as a compensation film to improve the viewing angle of an image. Compensation films (i.e. retardation films or phase difference films) are normally prepared from amorphous films that have a controlled level of birefringence prepared, for example, either by uniaxial stretching or by coating with discotic dyes. Suitable resins suggested for formation of compensation films by stretching include poly(vinyl alcohol)s, polycarbonates and sulfones. Compensation films prepared by treatment with dyes normally require highly transparent films having low birefringence such as TAC and cyclic olefin polymers.
In general, resin films as described above are prepared either by melt extrusion methods or by casting methods. Melt extrusion methods involve heating the resin until molten (approximate viscosity on the order of 100,000 cp), then applying the hot molten polymer to a highly polished metal band or drum with an extrusion die, cooling the film, and finally peeling the film from the metal support. For several reasons, however, films prepared by melt extrusion are generally not suitable for optical applications. Principal among these is the fact that melt extruded films exhibit a high degree of optical birefringence. In the case of highly substituted cellulose acetate, there is the additional problem of melting the polymer. Cellulose triacetate has a very high melting temperature of 270-300° C., and this is above the temperature where decomposition begins. Films have been formed by melt extrusion at lower temperatures by compounding cellulose acetate with various plasticizers as taught in U.S. Pat. No. 5,219,510 to Machell. However, the polymers described in U.S. Pat. No. 5,219,510 to Machell are not the fully substituted cellulose triacetate, but rather have a lesser degree of alkyl substitution or have propionate groups in place of some acetate groups. Even so, melt extruded films of cellulose acetate are known to exhibit poor flatness as noted in U.S. Pat. No. 5,753,140 to Shigenmura. For these reasons, melt extrusion methods are generally not practical for fabricating many resin films including cellulose triacetate films used to prepare protective covers and substrates in electronic displays. Rather, casting methods are generally preferred to manufacture these films.
Resin films for optical applications are manufactured almost exclusively by casting methods. Casting methods involve first dissolving the polymer in an appropriate solvent to form a dope having a high viscosity on the order of 50,000 cp, and then applying the viscous dope to a continuous highly polished metal band or drum through an extrusion die, partially drying the wet film, peeling the partially dried film from the metal support, and conveying the partially dried film through an oven to more completely remove solvent from the film. Cast films typically have a final dry thickness in the range of 40-200 microns. In general, thin films of less than 40 microns are very difficult to produce by casting methods due to the fragility of wet film during the peeling and drying processes. Films having a thickness of greater than 200 microns are also problematic to manufacture due to difficulties associated with the removal of solvent in the final drying step. Although the dissolution and drying steps of the casting method add complexity and expense, cast films generally have better optical properties when compared to films prepared by melt extrusion methods and, moreover, problems related to decomposition associated with exposure to high temperature are avoided.
Examples of optical films prepared by casting methods include: (1) Cellulose acetate sheets used to prepare light polarizing films as disclosed in U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289 to Cael as well as more recent disclosures in U.S. Patent Application 2001/0039319 A1 to Harita and U.S. Patent Application 2002/001700 A1 to Sanefuji; (2) Cellulose triacetate sheets used for protective covers for light polarizing films as disclosed in U.S. Pat. No. 5,695,694 to Iwata; (3) Polycarbonate sheets used for protective covers for light polarizing films or for retardation plates as disclosed in U.S. Pat. No. 5,818,559 to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani; and (4) Polyethersulfone sheets used for protective covers for light polarizing films or for retardation plates as disclosed in U.S. Pat. Nos. 5,759,449 and 5,958,305 both to Shiro.
Despite the wide use of the casting method to manufacture optical films, however, there are a number of disadvantages to casting technology. One disadvantage is that cast films have significant optical birefringence. Birefringence in cast or coated films arises from orientation of polymers during the manufacturing operations. This molecular orientation causes indices of refraction within the plane of the film to be measurably different. In-plane birefringence is the difference between these indices of refraction in perpendicular directions within the plane of the film. The absolute value of birefringence multiplied by the film thickness is defined as in-plane retardation. Therefore, in-plane retardation is a measure of molecular anisotropy within the plane of the film.
During a casting process, molecular orientation may arise from a number of sources including shear of the dope in the die, shear of the dope by the metal support during application, shear of the partially dried film during the peeling step, and shear of the free-standing film during conveyance through the final drying step. These shear forces orient the polymer molecules and ultimately give rise to undesirably high birefringence or retardation values. To minimize shear and obtain the lowest birefringence films, casting processes are typically operated at very low line speeds of 1-15 m/min as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Slower line speeds generally produce the highest quality films.
Although films prepared by casting methods have lower birefringence compared to films prepared by melt extrusion methods, birefringence remains objectionably high. For example, cellulose triacetate films prepared by casting methods exhibit in-plane retardation of 7 nanometers (nm) for light in the visible spectrum as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Polycarbonate films prepared by casting methods exhibit in-plane retardation of 17 nm as disclosed in U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani. U.S. Patent Application Publication 2001/0039319 A1 to Harita claims that color irregularities in stretched cellulose acetate sheets are reduced when the difference in retardation between widthwise positions within the film is less than 5 nm in the original unstretched film.
For many applications of optical films, low in-plane retardation values are desirable. In particular, values of in-plane retardation of less than 10 nm are preferred.
Commonly-assigned U.S. Patent Application Publications 2003/0215658A, 2003/0215621A, 2003/0215608A, 2003/0215583A, 2003/0215582A, 2003/0215581A, and 2003/0214715A describe a coating method to prepare resin films having low birefringence that are suitable for optical applications. The resin films are applied onto a discontinuous, removable carrier substrate from lower viscosity polymer solutions than are normally used to prepare cast films.
Another drawback to the casting method is the inability to accurately apply multiple layers. As noted in U.S. Pat. No. 5,256,357 to Hayward, conventional multi-slot casting dies create unacceptably non-uniform films. In particular, line and streak non-uniformity is greater than 5% with prior art devices. Acceptable two layer films may be prepared by employing special die lip designs as taught in U.S. Pat. No. 5,256,357 to Hayward, but the die designs are complex and may be impractical for applying more than two layers simultaneously.
Another drawback to the casting method is the restrictions on the viscosity of the dope. In casting practice, the viscosity of dope is on the order of 50,000 cp. For example, U.S. Pat. No. 5,256,357 to Hayward describes practical casting examples using dopes with a viscosity of 100,000 cp. In general, cast films prepared with lower viscosity dopes are known to produce non-uniform films as noted for example in U.S. Pat. No. 5,695,694 to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowest viscosity dopes used to prepare casting samples are approximately 10,000 cp. At these high viscosity values, however, casting dopes are difficult to filter and de-gas. While fibers and larger debris may be removed, softer materials such as polymer slugs are more difficult to filter at the high pressures found in dope delivery systems. Particulate and bubble artifacts create conspicuous inclusion defects as well as streaks which may result in substantial waste.
In addition, the casting method can be relatively inflexible with respect to product changes. Because casting requires high viscosity dopes, changing product formulations requires extensive down time for cleaning delivery systems to eliminate the possibility of contamination. Particularly problematic are formulation changes involving incompatible polymers and solvents. In fact, formulation changes are so time consuming and expensive with the casting method that most production machines are dedicated exclusively to producing only one film type.
Cast films may exhibit undesirable cockle or wrinkles. Thinner films are especially vulnerable to dimensional artifacts either during the peeling and drying steps of the casting process or during subsequent handling of the film. Very thin films are difficult to handle during this lamination process without wrinkling. In addition, many cast films may naturally become distorted over time due to the effects of moisture.
For optical films, good dimensional stability is necessary during storage as well as during subsequent fabrication of polarizer plates. In addition, resin films used in protective cover sheets for polarizer plates are susceptible to scratch and abrasion, as well as the accumulation of dirt and dust, during the manufacture and handling of the cover sheet. The preparation of high quality polarizer plates for display applications requires that the protective cover sheet be free of defects due to physical damage or the deposition of dirt and dust.
It would be very advantageous to avoid the need for saponification of protective cover sheets in the preparation of polarizer plates from resin films which requires a lamination process involving pretreatment in an alkali bath and then application of adhesives, pressure, and high temperatures. Avoiding such a saponification operation would improve both productivity and reduce the necessary conveyance and handling of the sheets. Although advantageous for protective cover sheets in general, this would be especially desirable for relatively thinner protective cover sheets.